Patent GB-806842-A (GB)
Title: Improvements in or relating to systems for processing recorded information
IPC: G11B20/22
Filed: 1956-06-20
Abstract: 806,842. Electric correspondence control. ELECTRO-MECHANICAL RESEARCH Inc. June 20, 1956 [June 21, 1955], No. 19094/56. Class 40 (1). [Also in Group XL (c)] A system for processing a timemodulated intelligence signal recorded with a reference frequency signal comprises a reference frequency discriminator tuned to the reference frequency to derive an error signal representing record speed variations, a tunable frequency discriminator for demodulating the intelligence signal and means for coupling the error signal with the tunable frequency discriminator to tune it in proportion to record speed variations to eliminate errors in the intelligence signal caused by such speed variations. As shown in Fig. 1, the reference frequency signal and a plurality of intelligence signals of different frequencies from a play-back unit 10 are separated by band-pass filters 15 and each is passed through an amplifier 16 and limiter 17, the limiter converting the sinusoidal signal output of the amplifier 16 into a squarewave signal of alternating polarity. The squarewave signal from the limiter 17 is differentiated and used to actuate a pulse generator 18 which produces a pulse coinciding with the beginning of each cycle of the square wave signal. The time spacing of the pulses is inversely proportional to the frequency of the signal. The pulses from the generator 18 are applied to a discriminator 19 consisting of a multivibrator which has a stable state and an unstable state, the return to the stable state occurring after a substantially fixed interval. Thus the multivibrator produces a negative pulse of constant time duration and fixed amplitude followed by a positive pulse of constant amplitude but of a duration which varies according to the frequency variations of the signal. These output pulses from the multivibrator are passed through a low-pass filter 20 which integrates them to produce an amplitude-modulated signal which is applied to an amplifier 21. The error signal produced by demodulating the reference frequency signal is supplied to the discriminators for the intelligence signals to vary the duration of the negative pulses produced by the multivibrators in such a way that the unwanted speed variations are eliminated. The error signal is coupled to each intelligence discriminator through phase adjusting circuits which can be adjusted to compensate for the difference in delay times between the reference channel and the intelligence channel and through a cathode follower circuit 31 the gain of which is adjustable to vary the amplitude of the signal applied to the frequency discriminator to adjust the degree of compensation. In a modification to reduce the effects of non-linearity in the discriminator of the reference frequency channel a negative feed-back loop is provided between the amplifier 21 and the discriminator 19 of the reference frequency channel. The multivibrator, Fig. 2, comprises a normally non- conducting pentode 35 and a normally conducting pentode 36. The anode of the pentode 35 is coupled by a diode 38, a cathode follower 40 and a capacitor 41 'to the control grid of the pentode 36 and the anode of the pentode 36 is coupled via a resistor-capacitor combination 50, 51 to the control grid of the pentode 35. For each square wave from the limiter, the pulse generator 18 produces a sharply falling negative pulse which is fed to the anode of the pentode 35. So long as this pulse is more positive than the junction of resistors 43, 44 the pulse is conducted through diode 38 to the grid circuit of cathode follower 40 and renders the cathode follower non-conductive. This causes the cathode voltage of the cathode follower and hence the voltage on the output line 48 to drop sharply and also drives the pentode 36 to cut-off. The rise in the anode voltage of the pentode 36 drives the pentode 35 to saturation and causes its anode potential to drop to maintain the cathode follower cut-off. The potential applied to the cathode follower is limited by the diode 38 to a fixed value. The cathode follower 40 and the pentode 36 remain cut-off for a period which is determined by the time taken by the capacitor 41 to recharge through resistors 56 and 57 to the bias potential applied to terminal 55 and when the pentode 36 starts to conduct again the circuit reverts to its stable state.